tDCS and Speech Therapy in Aphasia Treatment: A Multicenter Comparative Study of Efficacy

Introduction

Aphasia is a debilitating consequence of cerebrovascular accidents (CVAs) and traumatic brain injury (TBI) that significantly impacts patients’ communication abilities, independence, social participation, and overall quality of life [1]. Among the common sequelae of brain injury are speech and language impairments, which severely compromise patients’ communication abilities and quality of life [2]. With increasing survival rates following stroke and TBI due to advances in emergency medicine, the prevalence and public health burden of chronic aphasia have grown considerably, making effective rehabilitation interventions critically important [3]. Despite improvements in traditional speech-language therapy (SLT), recovery often remains incomplete, necessitating novel strategies to enhance language outcomes. Recent evidence suggests that adjunctive neuromodulation techniques, such as transcranial direct current stimulation (tDCS), can overcome existing therapeutic limitations by promoting neuroplasticity and optimizing cortical reorganization [4–6].

The tDCS, a non-invasive neuromodulation technique, shows promise in enhancing language recovery by facilitating cortical neuroplasticity and cognitive processes supporting speech and language functions [7,8]. However, substantial variability and inconsistencies persist in existing literature regarding stimulation targets, protocols, and patient responsiveness, resulting in unresolved debates and limiting clinical translation. Although traditional language areas (eg, Broca’s and Wernicke’s regions) are commonly targeted, recent evidence suggests that stimulation of the dorsolateral prefrontal cortex (DLPFC) can facilitate broader cognitive control processes, executive functions, and neural network reorganization crucial for language recovery, particularly verbal fluency, attention, and working memory. It can enhance language rehabilitation indirectly through improved executive and cognitive processes supporting language functions [9,10].

By applying a low-intensity direct current through the scalp, tDCS influences neuronal activity, fostering rehabilitation in brain-injured patients. Clinical studies indicate that tDCS accelerates recovery and improves the outcomes of speech therapy by enhancing cortical reorganization and facilitating the restoration of speech and language functions [11]. However, few randomized controlled trials (RCTs) have systematically assessed the efficacy of tDCS in patients with aphasia due to CVAs or TBIs [12].

Aphasia affects approximately 21–38% of stroke survivors and is also prevalent among TBI patients [13]. Traditional rehabilitation strategies, such as SLT, provide variable outcomes, often requiring long treatment periods [14]. Emerging evidence suggests that neuromodulation techniques like tDCS can enhance the brain’s capacity for reorganization, potentially expediting recovery and improving the effectiveness of conventional therapies.

Recently, non-invasive cortical stimulation has received considerable attention as a promising treatment for patients with aphasia caused by CNS damage, but more often in the course of CVA than TBI, as well as more often with the use of transcranial stimulation using rTMS (repetitive transcranial magnetic stimulation) than tDCS [15]. Although previous studies have explored the combination of tDCS and SLT in aphasia rehabilitation, the present study has several novel aspects. This trial features a multicenter design with a relatively larger and diverse patient cohort, encompassing individuals with aphasia secondary to both stroke and TBI, thus enhancing generalizability. Secondly, unlike most studies focusing on classical language areas, we uniquely targeted the DLPFC, emphasizing cognitive and executive functions crucial to language recovery. Finally, our study directly compares tDCS as monotherapy versus combined therapy with SLT, enabling clearer insights into the additive effects of neurostimulation.

The rationale for this study stems from the need to bridge the gap in aphasia rehabilitation by combining tDCS with standard therapeutic approaches. While existing studies had promising outcomes, the heterogeneity in protocols, stimulation parameters, and target populations necessitates further research. This multicenter trial seeks to establish robust evidence regarding the clinical utility of tDCS for aphasia treatment in a controlled and systematic manner.

The primary objective of this study was to assess the quantitative and qualitative improvements in language function following tDCS in patients with aphasia resulting from brain injury. This study hypothesized that: (1) tDCS (alone or combined with SLT) improves speech comprehension and expression significantly more than SLT alone; (2) combined tDCS and SLT intervention results in greater improvement in reading and writing abilities compared to either treatment alone; (3) combined intervention demonstrates additive or synergistic effects compared to tDCS or SLT alone; (4) certain clinical or demographic subgroups (eg, stroke vs TBI patients, younger vs older patients) respond better to tDCS interventions.

Material and Methods

STUDY DESIGN AND PARTICIPANTS:

This study was designed as a prospective, multicenter, randomized controlled trial conducted between April 2018 and January 2021. The trial involved 3 neurorehabilitation clinics specializing in aphasia treatment following brain injury, including AFA-MED Neurologopedic Centers in Żary, Szczecin, and Łódź (Poland), as well as a neurorehabilitation facility in Międzywodzie (Poland), which hosted patients for intensive therapeutic programs. The present study does not require registration in the WHO International Clinical Trial Registry Platform as it involves original research employing non-pharmacological interventions and constitutes a non-clinical study design.

STUDY ETHICS:

Ethics approval was secured from the Bioethics Committee at Wrocław Medical University (approval no. KB-805/2018). Written informed consent was obtained from all participants or their legal representatives. Participants were informed about the study’s objectives, potential risks, and their right to withdraw without consequences. Patient confidentiality was maintained by anonymizing data and securely storing sensitive information, ensuring compliance with data protection regulations. The study protocol was prepared in accordance with the requirements of the Declaration of Helsinki and Good Clinical Practice (GCP). All guidelines of the European Speech and Language Therapy Association (ESLTA) were met during the study.

PATIENTS’ QUALIFICATION:

Patients aged 18 years and older diagnosed with aphasia following ischemic or hemorrhagic stroke or TBI and who were in stable medical condition were considered eligible. These criteria were applied during the initial screening based on medical records, clinical assessments, and neuroimaging confirmation of diagnosis by a neurologist or neurosurgeon. The exclusion criteria considering other etiologies (toxic, metabolic, degenerative, idiopathic), severe cognitive impairment, medical or psychiatric conditions interfering with compliance, contraindications to tDCS (eg, metal implants, pregnancy, tumors), and refusal or inability to provide informed consent were systematically assessed during comprehensive clinical interviews and examinations. Although inclusion of both stroke and TBI patients introduced clinical heterogeneity, baseline demographic and clinical characteristics were carefully analyzed after randomization, confirming comparability across groups and ensuring balanced distribution of both patient types within each treatment arm.

RANDOMIZATION AND ALLOCATION:

Of the 180 individuals initially screened for eligibility, 134 met the inclusion criteria and were invited to participate. During the recruitment process, 44 individuals declined participation due to personal reasons or logistical challenges (n=24) as well as withdrawal of consent (n=11) or inability to adhere to the treatment protocol (n=9). Subsequently, 90 participants consented to enroll in the study and underwent randomization. A total of 90 patients diagnosed with aphasia after stroke or TBI were enrolled and randomly assigned to 1 of 3 intervention groups: Group I with 30 patients receiving tDCS intervention alone, Group II with 30 patients undergoing standard SLT, and Group III with 30 patients receiving combined tDCS and SLT. Figure 1 presents a study flowchart.

To account for potential variability due to differences in aphasia profiles associated with stroke and TBI, participants were randomized in a 1: 1: 1 ratio using a computer-generated allocation sequence (random.org). Allocation concealment was ensured using sequentially numbered, opaque, sealed envelopes, prepared independently by a researcher uninvolved in patient recruitment or treatment delivery. Envelopes were opened only after participants provided informed consent and completed baseline assessments, thereby maintaining concealment until treatment initiation. Baseline demographic and clinical characteristics were analyzed after randomization, confirming comparability across intervention groups, with no significant differences identified.

STUDY PROCEDURE:

The study followed a single-blind design, with unaware of their assigned intervention, but clinicians administering tDCS or SLT were not blinded. tDCS sessions for Groups I and III were conducted using a Soterix Medical 2-channel stimulator. Electrodes (rubber electrodes placed within saline-soaked sponge covers) measuring 5×7 cm (35 cm2) were used. The anode was placed over the left dorsolateral prefrontal cortex (DLPFC, F3), and the cathode was positioned over the right prefrontal cortex (F4) according to the international 10–20 EEG electrode placement system. The stimulation parameters and protocol followed standardized guidelines provided by Charvet et al (2015) for conducting tDCS in clinical trials [16]. Each stimulation session lasted 30–40 minutes, with current intensities ranging from 1.0 to 1.5 mA, depending on individual patient tolerance. Sessions were administered 7 times per month (approximately twice weekly) over a 5-month period, totaling 35 sessions.

SLT for Groups II and III involved standardized, weekly 45-minute sessions administered consistently across all study sites for 5 months (20 sessions in total). Therapy protocols included exercises aimed at enhancing speech production (eg, naming tasks, repetition, articulation practice), auditory comprehension (eg, following verbal instructions, answering questions), reading (eg, text comprehension, word-to-picture matching), and writing (eg, copying words, constructing sentences) [17]. Although exercises were individualized based on severity and specific language deficits, all clinicians followed a structured therapy framework with standardized procedures and therapy goals established before study initiation to ensure consistency across sites.

Adherence to scheduled intervention sessions was monitored using attendance logs maintained by therapists at each participating site. Attendance was reviewed weekly by the study coordinator, and any missed sessions were promptly addressed. Although 9 individuals were excluded during initial screening due to anticipated inability to adhere to the treatment schedule, adherence monitoring ensured complete compliance among all 90 enrolled participants who successfully completed their assigned protocols.

Potential confounding factors were carefully controlled to ensure reliability of our findings. Adherence to treatment was strictly monitored through attendance logs maintained by therapists and reviewed weekly by the study coordinator, with missed sessions promptly rescheduled. Complete adherence was achieved among the enrolled participants, minimizing adherence-related variability. Moreover, SLT was standardized across all participating sites, with therapists following structured, predefined therapy protocols and training sessions conducted before the study to ensure consistency. Regular supervisory meetings were held to reinforce protocol adherence and minimize inter-therapist variability, further strengthening control over confounding influences.

MEASURED OUTCOMES:

Primary outcomes were assessed using the Frenchay Aphasia Screening Test (FAST) to evaluate verbal expression, comprehension, reading, and writing [18]. Severity of aphasia was graded using the Aphasia Evaluation Scale (SODA) [19], which assessed verbal fluency and naming abilities. Auditory comprehension and language processing were quantified using the Token Test (TT) [20]. Secondary outcomes included improvements in functional independence, assessed by communication performance in daily activities, as well as patient satisfaction and compliance through self-reported post-intervention questionnaires.

RESEARCH TOOLS:

The Frenchay Aphasia Screening Test (FAST) was employed to evaluate key language functions, including speech comprehension, verbal expression, reading, and writing. This tool provided a comprehensive yet time-efficient assessment of aphasia severity by quantifying deficits across different domains of communication. FAST is widely recognized for its reliability in detecting language impairments in patients with neurological disorders and is frequently used in clinical and research settings to monitor treatment progress [18].

The Aphasia Evaluation Scale (SODA in Polish) was utilized to assess the qualitative and quantitative aspects of aphasia severity and dynamics. This tool measures various components of language function, such as verbal fluency, naming abilities, and the overall impact of aphasia on communication. SODA is particularly valuable for tracking changes in aphasia symptoms over time and identifying subtle improvements or deteriorations in language function following therapeutic interventions [19].

The Token Test (TT) served as a core instrument for assessing auditory comprehension and language processing in patients with aphasia. By requiring participants to follow verbal instructions involving objects of different shapes, sizes, and colors, TT provided a sensitive measure of auditory processing and the ability to understand complex verbal commands. This test is essential for identifying deficits in language comprehension and is often used to distinguish between different types of aphasia based on comprehension impairments [20].

SAMPLE SIZE:

The sample size for this study was calculated using G*Power 3.1 software. We conducted a priori power analysis based on detecting a moderate to large effect size (f=0.35, Cohen’s d≈0.7), consistent with previous literature on tDCS in aphasia treatment. Assuming an alpha level of 0.05, statistical power of 80%, and accounting for an approximately 10% potential dropout rate, we calculated that a minimum sample of 27 participants per group was required (81 participants total). We conservatively rounded this number up to 90 participants (30 per group) to ensure robustness and account for possible attrition or missing data, thereby balancing statistical rigor with practical feasibility. The power analysis aimed for 80% power to detect a clinically meaningful 15–20% difference in language function improvements between groups, based on previous studies of aphasia interventions. We set the significance level at p<0.05 and accounted for a possible dropout rate.

STATISTICAL ANALYSIS:

Statistical analysis was performed using STATISTICA v13.3 by TIBCO Software, Inc. The normality of data distribution was assessed using the Shapiro-Wilk test, revealing significant departures from normality (p<0.05). Consequently, non-parametric tests were selected for subsequent analyses. Specifically, the Kruskal-Wallis test was chosen for comparisons among 3 independent groups due to its suitability for ordinal and continuous variables not meeting parametric assumptions. For pairwise comparisons between 2 independent groups, the Mann-Whitney U test was used, as it is robust against non-normal distributions and appropriate for continuous, non-normally distributed data. Within-group comparisons of pre- and post-intervention outcomes were conducted using the Wilcoxon signed-rank test, due to its effectiveness in analyzing paired, ordinal, or continuous data without normal distribution assumptions. Pearson’s chi-square test was applied to compare categorical variables such as gender and employment status across groups. Lastly, Spearman’s rank correlation was utilized to evaluate relationships between clinical improvement and baseline characteristics due to its appropriateness for ordinal and non-normally distributed continuous data. Statistical significance was set at p<0.05.

Results

PATIENTS’ CHARACTERISTICS:

A total of 90 patients were enrolled in the study, with an even distribution across the 3 intervention groups. The final cohort consisted of 56 men (62.2%) and 34 women (37.8%), with a mean age of 53.6 years (SD±16.1; range 18–82 years). No significant differences in demographic variables, including age, gender, or educational background, were observed between the groups (p>0.05), ensuring comparability across the study arms.

Baseline functional assessments, including FAST, SODA, and TT scores, revealed no statistically significant differences among the groups (p>0.05). This indicates that the degree of aphasia and overall neurological impairment was comparable at the start of the intervention, supporting the validity of the randomized design and the reliability of subsequent outcome comparisons. Table 1 provides details of patient characteristics.

RESEARCH HYPOTHESES:

Our findings directly addressed our initial hypotheses. In line with hypothesis 1, tDCS (both alone and combined with SLT) produced significantly greater improvements in speech comprehension and expression compared to SLT alone (FAST and SODA results, Tables 2 and 3). Supporting hypothesis 2, the combined intervention (tDCS+SLT) demonstrated greater improvements in reading and writing abilities, as evidenced by significantly higher FAST scores than either treatment alone. Our hypothesis 3 regarding additive or synergistic effects of combined intervention was partially supported; combined therapy consistently yielded greater improvements compared to monotherapies, although statistical significance was not achieved in all comparisons. Lastly, hypothesis 4 about clinical or demographic subgroup responsiveness was not supported, as no significant differences were observed based on stroke vs TBI etiology or patient age.

LANGUAGE FUNCTIONS:

Baseline and post-intervention scores from the Frenchay Aphasia Screening Test (FAST) were analyzed to evaluate improvements in verbal expression, comprehension, reading, and writing across the 3 groups. At baseline, no significant differences in FAST scores were observed between Group I (tDCS), Group II (SLT), and Group III (tDCS+SLT) (p=0.78), confirming comparability across interventions. After 5 months of therapy, all groups demonstrated significant improvements in FAST scores (p<0.001 for all groups). The greatest improvement was observed in Group III (combined tDCS+SLT), with an average score increase of 32.6% compared to baseline. Group I (tDCS only) showed a 24.3% improvement, while Group II (SLT only) had a 17.8% increase.

Pairwise post hoc analysis revealed that the difference between Group III and II was statistically significant (p=0.009), indicating superior outcomes in the combined intervention group. The comparison between Group I and II also demonstrated significance (p=0.04), suggesting that tDCS alone produced better results than SLT alone. However, the difference between Group I and III approached but did not reach statistical significance (p=0.07), suggesting the superiority of combined therapy (Table 2).

APHASIA SEVERITY:

The Aphasia Evaluation Scale (SODA in Polish) was used to assess the severity and dynamics of aphasia, focusing on verbal fluency, object naming, and language structure. Baseline assessments revealed no significant differences in SODA scores among the 3 intervention groups (p=0.81). Post-intervention analysis demonstrated significant improvements in SODA scores across all groups (p<0.001 for all). The largest improvement was observed in Group III (tDCS+SLT), with a 29.5% increase in scores from baseline. Group I (tDCS only) showed a 21.8% improvement, while Group II (SLT only) had a 15.2% increase.

Between-group comparisons showed that Group III achieved significantly greater improvements than Group II (p=0.01). The difference between Group I and Group II was also significant (p=0.03), reflecting the efficacy of tDCS as a standalone intervention. The difference between Group I and III approached statistical significance (p=0.06), suggesting a trend favoring the combined intervention (Table 3).

AUDITORY PROCESSING:

The Token Test (TT) was administered to evaluate auditory comprehension and the ability to follow complex verbal instructions, a key indicator of aphasia severity and recovery. Baseline assessments showed no statistically significant differences in TT scores among the 3 groups (p=0.76), confirming group comparability at the start of the intervention. Post-intervention results revealed significant improvements across all groups (p<0.001 for each). The most pronounced gains were observed in Group III (tDCS+SLT), with a 34.2% increase in TT scores from baseline. Group I (tDCS only) showed a 26.7% improvement, while Group II (SLT only) demonstrated a 19.5% increase.

Between-group comparisons revealed that Group III achieved significantly greater improvements than Group II (p=0.008). The difference between Group I and II was also significant (p=0.02), reflecting the beneficial effects of tDCS on auditory comprehension. The comparison between Group I and III approached but did not reach statistical significance (p=0.07), suggesting a trend in favor of the combined intervention (Table 4).

FUNCTIONAL INDEPENDENCE AND DAILY COMMUNICATION:

In addition to standardized tests (FAST, SODA, and TT), patient-reported outcomes regarding functional independence and daily communication were collected using post-intervention questionnaires. These assessments measured patients’ perceived improvements in everyday communication, social interactions, and confidence in verbal expression.

Results indicated that 78% of patients in Group III (tDCS+SLT) reported significant improvements in their ability to engage in daily conversations, compared to 63% in Group I (tDCS only) and 52% in Group II (SLT only). Group III participants reported greater confidence in initiating conversations and responding to complex verbal cues (p=0.01 for Group III vs II). Self-reported social reintegration scores improved by 35% in Group III, 27% in Group I, and 19% in Group II (p=0.03 between Group II and III) (Table 5).

APHASIA IMPROVEMENT BASED ON CLINICAL VARIABLES:

Regarding treatment effects based on changes in the degree of aphasia in groups differing in clinical characteristics revealed no statistically significant association between treatment effects and type of main disease and number of stroke incidents (p>0.05). This means that the results obtained are not dependent on the type of brain injury (stroke or TBI) or the number of stroke or traumatic incidents (one-time or consecutive episode) (Table 6).

ADVERSE EFFECTS AND TOLERABILITY OF TDCS:

Adverse effects were minimal and consisted primarily of mild scalp irritation (reported by 13% of participants receiving tDCS) and transient headaches (8%). Scalp irritation was typically reported as slight tingling or mild discomfort at the electrode placement sites, resolving spontaneously within a few minutes after session completion. Transient headaches were mild in intensity, self-limiting, and resolved within 1–2 hours after the intervention without any additional medical care. Importantly, no serious or persistent adverse events occurred, and no participants discontinued participation in the study due to adverse effects.

No severe adverse events were reported, and 92% of participants expressed willingness to undergo further tDCS sessions. The high tolerability and minimal adverse effects highlight the safety and feasibility of tDCS as a therapeutic intervention. The low dropout rate supports the acceptability of tDCS, particularly when combined with conventional SLT (Table 7).

Discussion

STUDY LIMITATIONS:

While demonstrating significant improvements in aphasia recovery using tDCS and SLT, our study has several limitations. Firstly, the lack of a sham-control group can be a limitation but it was not included, as the primary aim of this study was to directly compare the clinical effectiveness of active interventions (tDCS alone, SLT alone, and their combination). Furthermore, the existing literature has consistently established superior outcomes of active tDCS compared to sham stimulation, providing adequate justification for omitting a placebo group. Secondly, the inclusion of patients with aphasia resulting from both stroke and TBI introduced heterogeneity in brain lesion characteristics, potentially influencing treatment responses and recovery trajectories. Thirdly, the follow-up duration of 5 months, although sufficient to detect immediate therapeutic effects, may not adequately capture long-term outcomes, necessitating extended observation periods in future research. Additionally, our relatively modest sample size (N=90), though adequate for detecting initial clinical effects, could restrict generalizability.

PRACTICAL IMPLICATIONS:

The findings of our study show tDCS is a valuable, non-invasive, and cost-effective tool for enhancing aphasia recovery. Its efficacy as both a standalone and adjunct therapy suggests that tDCS could complement traditional SLT, accelerating language improvements and reducing therapy duration. The high tolerability and minimal adverse effects observed reinforce the feasibility of integrating tDCS into routine outpatient neurorehabilitation. Clinicians can apply tDCS to patients making limited progress with conventional therapy, promoting further recovery and functional independence. By supporting personalized tDCS protocols and adapting procedure to individual needs, this approach holds significant potential for improving communication outcomes and enhancing patients’ quality of life.

FUTURE DIRECTIONS:

There is a need to explore the long-term effects of tDCS on aphasia recovery beyond the 5-month intervention period, providing insights into the durability of therapeutic benefits and potential relapse prevention. Investigations should aim to identify patient-specific factors, such as lesion location, aphasia type, and baseline cognitive function, that may predict optimal responsiveness to tDCS, facilitating the development of more targeted treatment plans. Additionally, there is a need to evaluate the efficacy of early tDCS intervention during the acute phase of stroke or TBI, as this may enhance neuroplasticity and promote faster recovery. Also, advancing research on personalized tDCS protocols that adapt stimulation parameters to the unique needs and recovery trajectories of individual patients could further improve therapeutic outcomes and expand the applicability of tDCS in clinical practice. Finally, including neuroimaging assessments would ensure a deeper understanding of the cortical mechanisms underpinning recovery. Future studies incorporating functional imaging would significantly enhance insights into the neural substrates of tDCS-induced language improvements.

Conclusions

The results of this multicenter randomized controlled trial demonstrate that tDCS, both as a standalone intervention and in combination with conventional SLT, significantly improved language function in patients with aphasia following stroke or TBI. Combined tDCS and SLT interventions led to notable improvements across all measured parameters, including verbal expression, comprehension, reading, and writing. Although our findings suggest potential additive benefits of integrating neurostimulation with standard rehabilitation methods, future studies specifically designed as superiority trials are needed to confirm these effects.